1. Field of the Invention
The present invention is directed generally towards a species transfer system. In particular, the present invention is designed to absorb a specific species, CO2, from large amounts of flue gas exiting a power plant, and transfer the species to a controlled outlet stream, and ultimately transforming the flue gas into a CO2 lean gas stream. The combustion of carbonaceous materials using air as an oxidant generates a combustion product having as major constituents CO2, N2 and moisture. There is a strong interest in removing CO2 from the power plant flue gas for sequestration and other purposes.
2. Description of the Related Art
Generally, both solid sorbents and liquid solutions, among other methods, can be used to separate a gaseous component from a gas mixture. For some applications, solid sorbents are more desirable than liquid solutions in separating the desired gaseous component. One of these applications is to remove CO2 from the flue gas emitted from power plants that use carbonaceous materials as fuel. The advantage of using solid sorbents for this application is the low energy consumption upon regeneration that improves the overall process efficiency significantly.
The process of removing CO2 from flue gas generally uses sorbent in two reacting systems: one for absorption and another for regeneration. Repeated cycling of the sorbent between the absorber and regenerator (cycling sorbent from spent, to regenerated) minimizes the need for additional sorbent into the system after the initial charge, if the equipment is carefully designed to reduce attrition of the sorbent.
In the absorber, the solid sorbent comes into contact with the flue gas, which contains a number of different components including CO2. In many different ways, a gas-sorbent contactor can be constructed using principles of the conventional moving bed or bubbling fluidized bed. Some mechanical mixing devices such as screw mixers, rotary kilns, and fabric filter bags also can be used to facilitate gas-sorbent contact. Yet, these conventional contactors are economically unfeasible and difficult to apply in practice to the absorption of CO2 from power plant flue gas for at least the following reasons: (i) the volume of the flue gas from a power plant is large—an 880 MWe coal-burning plant can generate flue gas at a rate of more than 120 million ft3/hr; (ii) the large volumetric flow rates of the flue gas essentially rules out the possibility of using a moving bed absorber, which requires the gas velocity to be below approximately 3 ft/s; and (iii) a large number of absorbers and regenerators would have to be installed and operated to handle the flue gas from a modern power plant at such low velocities.
Using a bubbling fluidized bed would cause the same problems as using a moving bed absorber and the regenerator, because the gas velocity in a bubbling bed needs to be below approximately 5 ft/s under most circumstances. Furthermore, the distributor used in a bubbling bed requires high velocity jets from the nozzles of the distributor, which has been proven to be one of the major sources for the attrition of the sorbent in the bed. As sorbents are relatively expensive, any excessive attrition can render the technology economically unfeasible. Therefore, a bubbling fluidized bed is not applicable with power plant systems.
A further complication in using solid sorbents for CO2 removal is the need for the gas to remain in contact with the sorbent for a few seconds even when the absorption kinetics are relatively fast. Such relatively long contact times makes it difficult to use a fabric filter as an effective contactor for the CO2 absorption, as the gas is in contact with the solid cake only for a fraction of a second when the gas flows through the fabric contactor.
Another issue that arises with absorption of CO2 in flue gas by solid sorbents is the large volume of CO2 generated from a power plant and relatively low absorption capacity per unit weight of the sorbent. For most sorbents, the absorption capacity (or the working capacity) is less than 10% of the mass of the sorbent. In other words, for each 10 pounds of sorbent fed, the amount of CO2 absorbed is less than 1 pound. As the CO2 generation rate is at least 1650 lbs for each megawatt of power generated from pulverized coal, it follows that for each megawatt generated, the sorbent required is about 15,000 lbs for 90% CO2 capture. Following the same logic, for an 880 MWe power plant, the solid sorbent required is more than 13 million lbs/hr. In a system where the sorbent is circulated, the amount of CO2 absorbed in the first pass is limited to about 3% depending upon the kinetics of absorption. With this limitation, a sorbent circulation rate of over 40 million lbs/hr is needed to absorb 90% CO2 in the flue gas, and the sorbent needs to be circulated multiple times inside the absorber to fully utilize the sorbent before attempting regeneration. Sorbents that exhibit high exothermic heat of reaction require even higher circulation rates to moderate temperature increase in the absorber.
It is impractical to use the sorbent in a once-through process. For a commercial process, the sorbent has to be reused multiple times through a regeneration process. Handling millions of pounds of sorbent circulating at a rate of tens of millions of lbs/hr is a serious challenge for existing reactors as absorbers and regenerators. The challenge lies both in designing the internals of the reactor, and in moving a large mass of sorbent between the absorber and regeneration assemblies.
Gas—solids (catalyst, sorbent) contact processes have been developed to remove pollutants such as sulfur oxides, nitrogen oxides and mercury from power plant flue gas. EP0174109 to Tolpin et al. discloses a process for particulates, nitrogen oxides and sulfur oxides, to be removed from power plant flue gases using adsorbent material that can be regenerated. Such processes are not applicable for the removal of CO2 from flue gas because CO2 constitutes a much larger percentage of flue gas (up to about 15 volume %) compared to ppmv and ppbv levels of pollutants. The high CO2 content in power plant flue gas requires that much larger quantities of solid sorbent be used in the removal process than the sorbent/catalyst quantities used for pollutant removal.
Another practice that has received attention is the use of multiple fixed bed vessels in which one-half of the vessels are absorbing CO2 while the other half are regenerating. As disclosed in U.S. Pat. No. 6,755,892 to Nalette et al., once the regeneration is complete, the flue gas flow is switched from absorbing vessel to a vessel that has completed regeneration. The periodic switching of inlet and outlet valves and the size of the fixed beds make the scheme feasible on a smaller scale, but impractical to treat flue gas from a power plant. Also, this process is highly inefficient in terms of energy usage. For example, when the sorbent is heated to regeneration temperature, the entire vessel must be heated, and when the sorbent is cooled down to absorption temperatures, the entire vessel must be cooled down.
US Patent Publication No. 2008/0119356 to Ryu et al. discloses in detail preparation methods and required properties of dry regenerable sorbent for CO2 capture from thermal power generation plants. US Patent Publication No. 2008/0119356 does not describe the method(s) for using the sorbent in a feasible commercial process.
At the Fourth Annual Conference on Carbon Capture & Sequestration, in Dry Regenerable Carbonate Sorbents for Capture of Carbon Dioxide from Flue Gas, Nelson et al. (paper 67, page 17 available at http://204.154.137.14/publications/proceedings/05/carbon-seq/Table % 20of % 20Contents.pdf) a highly conceptual scheme to separate CO2 from power plant flue gas is disclosed. The scheme uses a heated screw conveyer as a regenerator. The sorbent is lifted to higher elevations with the screw conveyor before flowing down into a cooler and a down-flowing absorber. In such a scheme, the sorbent is partially utilized as it passes through the absorber only once per cycle. This requires unnecessarily high sorbent circulation rates between the absorber and regenerator. With such poor utilization of sorbent, a significant amount of energy is lost in heating and cooling the sorbent. Also, no provision is made to moderate the absorption temperature, which will increase significantly with exothermic heat release, and such high absorption temperatures decrease the absorption capacity of the sorbent.
Another complicating factor in developing methods for large commercial systems is that the power plant flue gas pressure is low—around ambient pressure. Even though higher pressure flue gas is helpful and highly desirable in designing power plant scale gas-sorbent contact systems, it is economically impractical to attain such higher pressures due to the large volume of flue gas from power plants. Since a large amount of flue gas is released from a modern power plant (for a power plant of size 880 MWe, the flue gas flow rate is about 34,000 ft3/sec at ambient conditions), minimum power consumption becomes an important consideration in the design of an absorber to remove the CO2 from the power plant flue gas.
Among different gas-solids contactors, the moving bed contactor has the potential to achieve the lowest pressure drop in the absorber because larger particle sizes and low gas velocity can be utilized. However, such a low operating pressure drop is hard to achieve in practice. Because of the requirement to impregnate chemical agents to the particles to facilitate CO2 absorption, the particle size cannot be larger than 10 mm to achieve good chemical agent distribution with sufficient porosity and strength. The gas velocity cannot be too low (for example, less than approximately 3 ft/s) to avoid extreme absorber sizes. Under this circumstance, for a 2 second gas superficial residence time, the bed pressure drop is about 8 inches of water column. Taking into consideration the pressure drops in particle filtration baghouse and gas distribution, the total pressure drop for a moving bed absorber needs to be at least 25 inches of water column. For even a modest 300 MWe power plant, the required absorber cross-sectional area is over 4,000 square feet.
A bubbling bed requires an unusually high pressure drop. Under reasonable assumptions, the bubbling bed can use smaller particles (in the 3 mm range) with a gas superficial velocity of 3 ft/s and a bed height of 6 ft (for a 2 second residence time), and the pressure drop in the bed will be more than 80 inches of water column. Increasing the gas velocity for the bubbling bed will not help because, for a given gas residence time, an increase in the gas superficial velocity will increase the bed height without the benefit of decreasing the bed density. To reduce the pressure drop to 25 inches of water column, the gas superficial velocity has to be reduced to 0.4 ft/s, which would require an impractical 190 feet equivalent diameter vessel for a 300 MWe power plant.
It can be seen that a successful absorber and regeneration assembly to facilitate gas-sorbent contact and CO2 removal from the flue gas of a power plant needs to consider all of the above factors. A need yet exists for removing CO2 from the power plant flue gas for sequestration and other purposes. The present invention is directed to same, and resolves the above-mentioned barriers to a successful system.